The present invention relates to a preservative-bearing pad and method of using same to preserve wooden railroad cross ties and more particularly pertains to a treatment of the interface between the tie and the rail supporting tie plate.
Wooden railroad cross ties are made decay resistant by forcing a material such as creosote into the wood cell structure under relatively high pressure. This protects the exterior wood layer but the interior is not penetrated by the creosote and is subject to attack by decay fungi whenever the exterior layer is split, cracked, abraded away or otherwise structurally compromised. Penetration of the treated exterior is common after extended service use as a result of weathering and also as a result of what is known as spike kill and plate cut.
Weathering typically causes weather checking in the form of splits or cracks running in the direction of the wood grain. These cracks provide passages for moisture to travel under the tie plate supporting the rail and into the tie spike holes and checks.
Spike kill is the mechanical enlargement of spike holes caused by cyclical train loadings on the rail, spike and tie plate, the relative movement between these components eventually enlarging the spike hole and exposing untreated tie wood. This relative movement also causes plate cut, which is the wearing or cutting away of the tie at the interface between the tie plate and tie. This action tends to abrade away the treated exterior wood layer and expose the tie interior to the intrusion of moisture and wind borne fungi spores.
Soon after railroad ties are put into service moisture sites become established, particularly in the central portion of the interface between the tie and tie plate. This central portion never seems to dry. As a consequence, the presence of moisture and the temperature elevation brought on by exposure to the sun serve as an incubator for the growth of decay fungi. The natural balance of food, moisture and temperature accelerates destruction of the wood cells and exaggerates spike kill and plate cut in a cycle which eventually results in premature failure and costly replacement of the ties.
A variety of approaches have been taken in the prior art to extend the service life of wooden supports such as power poles or railroad cross ties. One area of concentration focuses on the formulations with which the wood is initially treated. U.S. Pat. No. 4,335,109 for example describes a composition which enhances the water repellency of the wood, while U.S. Pat. No. 1,620,152 provides a formulation with augmented fungicidal properties. Other efforts concentrate on the method of pretreatment to provide processes which force various preservative solutions deeper into the interior of the wood. U.S. Pat. No. 1,936,439 for example, describes a process in which ties are cross-drilled prior to impregnation with creosote under heat and pressure. The process of U.S. Pat. No. 4,202,494 encases a cross tie in polypropylene. These methods have the inherent disadvantage in that they do not lend themselves to the in situ treatment of railroad cross ties. U.S. Pat. No. 1,388,877 provides for the formation of a cavity within the wood structure that functions as a refillable reservoir for preservative solutions. While this reservoir may be filled or refilled in the field, the initial boring must be done prior to installation.
Other methods focus on the treatment of wood already installed or already in service and perhaps even already somewhat decayed. U.S. Pat. No. 4,048,353 and No. 4,269,875 describe methods by which railroad cross ties are bored and treated while U.S. Pat. No. 4,738,878 provides a process for injecting a preservative into the especially vulnerable area between the tie and tie plate. This latter method actually relies on existing cracks and fissures to promote a deeper penetration of preservative into the wood. The disadvantages associated with these methods is that they involve labor intensive operations and require special tools and equipment.
Another type of approach more commonly employed in the treatment of power poles involves the application of a wrap or bandage onto the surface of the wood. These wraps typically consist of a multi-layer structure including a layer of preservative engorged material, a moisture barrier and perhaps a strength imparting layer that holds the whole bandage together. U.S. Pat. No. 3,467,490 and U.S. Pat. No. 3,420,617 are representative examples of such technology. The disadvantages associated with this type of preservative treatment is the relatively high cost of manufacture of the bandages and the fact that the inert elements remain long after the active ingredients have been expended as these elements are not biodegradable. In addition, the prior art bandages are relatively thick and often bulky.